iio:light:tsl2563 move out of staging

This driver is simple, uses the latest interfaces and contains few if
any controversial elements.  All of its interfaces have been in place
for a long time now.  Hence let's move it out of staging.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Peter Meerwald <pmeerw@pmeerw.net>